Coatings for Metal Surfaces

ABSTRACT

An iron-containing metal surface, such as a brushed stainless steel, comprises a heat-set layer of a metal phosphate reaction product formed between a surface metal and phosphate precursor wherein the reaction product layer has an average thickness less than 2 micrometers. A metal surface, that resists retention of environmental stains or marks as well as resisting tarnish at elevated temperatures, is formed by forming a reaction product with the metal surface by heat setting a surface liquid layer containing a phosphate precursor such as a phosphate ester-containing solution or phosphoric acid solution, and optionally containing a phospho alumina precursor, onto the metal surface. An article coated in accordance with this invention is similar in appearance to a similar article without such a coating for maintaining original aesthetic properties.

CROSS-REFERENCE To RELATED APPLICATIONS

This application claims priority to U.S. patent application Ser. No. 14/124,895, filed Dec. 9, 2013, which is a U.S. National Phase of PCT/US2012/050826, filed Aug. 14, 2012, which claims priority to U.S. Provisional Patent Application 61/523,946, filed Aug. 16, 2011, and to U.S. Provisional Patent Application 61/562,637, filed Nov. 22, 2011, each incorporated by reference herein.

BACKGROUND OF THE INVENTION

This invention relates to inorganic protective systems for iron-containing metal substrates and more particularly relates to protective fillers or coatings applied to metal substrates particularly with heterogeneous surface microstructure, such as in brushed metal substrate surfaces, to inhibit fingerprint and stain retention, impart tarnish protection, be easy to clean, and maintain surface appearance.

Common metal surfaces, such as brushed surfaces, may be subject to exposure with materials that leave lasting marks on the surface. An example of such exposure is handling a metal article and leaving fingerprints on the article. Another example is application of materials such as paint, spray paint, markers, and the like that leave stains or other residue on a metal surface. Although fingerprint or stain resistant coatings are known, especially organic or polymeric coatings, such coated surfaces typically are soft, not uv resistant, not organic solvent resistant, and not tarnish resistant at elevated temperatures. Further, polymeric coatings (such as may be applied to consumer appliances) may have a noticeable thickness and have poor aesthetics. There is a need to provide a metal surface, especially a brushed metal (such as an iron-containing metal) surface having an heterogeneous surface microstructure, that is capable of easily removing foreign surface materials such as fingerprints, foodstuff, stains, spray paint, markers, and the like. Another need is to maintain the aesthetic appearance of such a metal surface, such that there is little if any difference between a coated and uncoated surface. Also, there is a need to provide an iron-containing metal surface that resists tarnish at elevated temperatures.

Metal substrates with heterogeneous surface microstructure such as brushed metal surfaces have series of valleys and plateaus (or peaks) on the metal surface. Typically, valleys and plateaus in a brushed surface will be oriented directionally. These valleys may be primarily responsible for poor fingerprint resistance and staining under common household use along with the need to use of aggressive and environmentally hazardous cleaning agents to remove them. Thus, treating a surface such that these valleys are filled with a stable, thin, optically transparent inorganic material in a practical and economical way is desirable. In addition, under elevated temperature or exposure to corrosive environments (such as coastal or marine environments including other salt environments), a change in appearance on a brushed surface primarily may be attributed to the accelerated degradation (corrosion or oxidation) of “valleys” of the brushed metal surface. The effect is more pronounced for deeper valleys of surfaces finished with coarser grit (e.g., <240) abrasive pad. Thus, certain stainless finishes are more susceptible to such degradation in the valleys. These valleys are chemically complex and are readily susceptible to oxidation or corrosion. Thus, in the case of oxidation at elevated temperatures (such as on a cooking range top), the valleys of a brushed stainless steel oxidize more rapidly compared to the “plateaus” with the surface appearance dominated by the oxidized valleys. Stainless 430 grade alloys are more susceptible to oxidation and corrosion compared to stainless 304 alloys, but they are much less expensive with lower chromium content. Enabling stainless 430 and other ferritic stainless alloys in common use to be used in more stressful environments will yield substantial economic savings and lower use of chromium. Appropriate surface engineering methods can be employed to substantially improve stainless 430 performance beyond uncoated stainless 304 performance levels.

A stable “filler” of the valleys can impart the desired surface properties. Such desired properties include corrosion resistance or resistance to staining by commonly used organic media or cleaning agents. Organic (or substantially organic) fillers such as polymeric materials (including polymer-silica hybrids) are typical and used extensively in commercial brushed stainless steel appliances, but their durability and inertness for common use is limited, and in particular they are susceptible to attack under uv radiation, which limits their use in outdoor applications, and to organic cleaning solvents such as ketones. Further, polymeric-based coatings may alter the surface appearance to form a duller or unattractive plastic-like appearance. Inorganic fillers and surface treatments can serve ideally to withstand harsh environments including organic solvents; however, the process economics must be favorable for continuous production, and the material should exhibit desired chemical, physical, elevated temperature stability, and barrier properties while maintaining aesthetics (e.g., the original surface appearance) of the uncoated brushed stainless steel substrate surface. Thus, an aspect of this invention is to produce a metal surface (such as an iron-containing metal surface) economically that is resistant to surface stains or discoloration (such as fingerprint resistance) that also is not susceptible to attack by organic solvents and to oxidizing or corrosive environments, such as heat exposure of 300-500° C.

Many, iron-containing metal surfaces, including brushed metal surfaces, will tarnish after exposure to heating such as up to 450 to 500° C., and tarnish typically is observed at temperature above 300° C. and becomes pronounced at 350 to 400° C. These temperatures are in a range typically experienced in common activities such as cooking. Tarnish is characterized by a change from a metallic luster finish to a typically yellowed or brown finish or dull appearance, and usually is attributed to oxidation. Such oxidation is enhanced in brushed metal surfaces compared to more relatively planar surfaces and the appearance is more significantly altered by loss of metal luster and discoloration.

There are several techniques to create a finished (e.g., brushed) metal substrate and the resulting surface microstructure can vary to a large degree with respect to the depth and breadth of valleys and the related chemistries. Rolled finishes (such as a mill finish or a patterned finish) are more stable and demonstrate better performance against corrosion compared to grit finished surfaces, although a mill or patterned finish surface will tarnish under harsh environments.

An aspect of this invention is the use of a phosphate material primarily derived from a metal surface to fill the valleys (either partially or fully) preferably together with a secondary passivation layer with a low surface energy and stable/inert surface chemistry. Another aspect of the invention is a method of phosphating to enable low cost, continuous production with an environmentally friendly chemical formulation.

Also, there is a need to provide barrier or protective coatings on easily oxidized or corroded metals such as carbon steel.

SUMMARY OF THE INVENTION

An iron-containing metal surface, such as a brushed stainless steel, comprises a heat-set layer of a metal phosphate reaction product formed between a surface metal and phosphate precursor wherein the reaction product layer has an average thickness less than 2 micrometers. A metal surface, that resists retention of environmental stains or marks as well as resisting tarnish at elevated temperatures, is formed by forming a reaction product with the metal surface by heat setting a surface liquid layer containing a phosphate precursor such as a phosphate ester-containing solution or phosphoric acid solution, and optionally containing a phospho alumina precursor, onto the metal surface. An article coated in accordance with this invention is similar in appearance to a similar article without such a coating for maintaining original aesthetic properties.

DESCRIPTION OF THE INVENTION

Generally, metal surfaces suitable for coating or treatment according to this invention include those surfaces that contain metal elements capable of forming metal phosphate bonds under the conditions experienced in this invention. Preferable metal surfaces include surfaces containing iron, chromium, nickel, and titanium, and alloys thereof. A preferable metal surface contains iron such as a steel. A preferable metal useful in this invention is stainless steel, an alloy containing iron and chromium. Under normal conditions, a thin layer of chromium oxide protects the surface from corrosion or oxidation of the underlying substrate (primarily iron). However, for many grades of stainless steels, at elevated temperatures such as above 200°, 400°, or 450° C., iron may oxidize to create a tarnish. Typically, a stainless steel containing more iron and less chromium will tarnish or corrode more easily. However, stainless steel grades containing more alloy constituents such as chromium, molybdenum, vanadium, and the like usually are more expensive. Thus, 304 stainless (SAE stainless steel designation) is more expensive than 430 stainless, but 304 stainless steel is used in more corrosive or oxidative environments. There is a substantial economic incentive to produce a lower grade stainless (such as 430) with a protective coating or treatment that permits a coated 430 stainless to be used in environments that now must use 304 stainless steel products. The tarnish protection of 304 stainless steel with a coating of this invention is substantially better than an untreated 304 stainless exposed to similar high temperatures for a given time period.

Metal surfaces that may be treated according to this invention preferably are stainless steels and especially brushed stainless steels. A “brushed” metal surface may be produced by abrading a metal surface with a suitable abrasion material such as a grit-containing material such as sandpaper, which typically aligns resulting striations directionally. A brushed surface is especially susceptible to fingerprint or stain retention. A brushed surface will appear dull or matte in comparison to a polished metal surface. Brushed surfaces may be designated by Standard Mill Surface Finishes as set by BS 1449, Part 4, and the Committee of Stainless Steel Producers, American Iron and Steel Institute. Common finishes for stainless steel include #3, #4, and #6. Stainless steel with a #4 (satin) finish is typical. Also, a roller may be used to form a brushed or heterogeneous metal surface. Typical finishes may have Ra of 0.2 to 1.5 micrometers, more typically 0.4 to 0.6 micrometers and preferably 0.4 to 0.5 micrometers.

In an aspect of this invention, a layer of an inorganic metal phosphate reaction product is formed on an iron-containing metal surface, especially within microstructure valleys on such surface. This reaction product is formed at elevated temperatures from the surface metal and a phosphate precursor and primarily will be an iron phosphate.

In an aspect of this invention, a layer of a phosphate precursor solution such as a phosphoric acid containing solution or a phosphate ester solution is heat-set to form a protective layer on a metal surface. Such a protective layer typically is smooth to the touch and does not alter the visual appearance of the metal surface. Typically, in accordance with this invention, a layer or film containing or derived from a phosphoric acid or phosphate ester is coated and heat-set onto a metal substrate such as a stainless steel. Typically, a phosphate precursor solution is applied to a metal surface by dip, spray, or roll coating, although other suitable application methods may be used. A phosphate ester-containing material is a solution of at least one phosphate ester or a precursor species such as phosphoric acid in a suitable solvent that is compatible with such phosphate ester or precursor (such as an alcohol) and, preferably, is capable of wetting the metal surface. A preferable phosphate precursor is a phosphate ester-containing solution. As used in this invention, a phosphate ester-containing solution includes reaction products between phosphoric acid and a solvent such as an alcohol to form an ester. Thus, an ethanol solution containing phosphoric acid may include presence of a phosphate ethyl ester. Such a “phosphate ester-containing solution” may be termed a “phosphate ester derived solution.” Further, such a phosphate precursor solution may be considered as having a phosphorus-containing compound capable of forming a metal to oxygen to phosphorus (M-O-P) bond through a heat set layer. Further, a solution containing a phosphating species that acts in a similar way under the conditions used in this invention to form a treated or coated metal surface as described in this invention is considered phosphate ester-containing solution. A phosphate ester useful in this invention typically may be an alkyl (typically C₁-C₈) ester of a phosphoric acid or alkyl acid phosphate such as a monoalkyl acid phosphate, dialkyl acid phosphate, or dialkyl acid pyrophosphate. A typical phosphate ester useful in this invention is a C₁-C₄ phosphate ester such as a methyl, ethyl, propyl, or butyl phosphate. A preferable phosphate ester is ethyl phosphate, which may be derived from phosphoric acid and ethanol.

As used in this invention, a “layer” of a phosphate reaction product denotes coverage of a metal surface with the reaction product in which the thickness of such coverage may vary over the surface area. Thus, a surface having valleys and plateaus may have a thicker amount of reaction product in the valleys and a thinner amount on the plateaus. A typical layer of a phosphate reaction product of this invention has a thickness of less than 2 micrometers (and more typically less than 1 micron) averaged over the valleys and plateaus. In a typical covered brushed surface of this invention, at least 25 vol. % of the top 2 microns of the surface is metal and at least 25 vol. % of the top 2 microns of the surface is phosphate reaction product.

An aspect of this invention is forming on a metal surface a primary layer of inorganic metal phosphates derived from heat-treating a solution containing phosphate esters or precursors to phosphate esters. Another aspect of this invention is prior to such heat-treating combining a solution of phosphate esters or precursors thereof with a precursor solution of a phospho alumina material such that a primary layer contains inorganic phosphorus and aluminum species.

Also, an aspect of the invention is forming inorganic metal phosphates (which may be within valleys in brushed metal surfaces) from a source of phosphorus contained in a precursor solution and metal derived from the metal surface. Such source of phosphorus may be a phosphorus-containing compound capable of forming a metal to oxygen to phosphorus (M-O-P) bond during formation of a heat set layer. If a phospho alumina precursor solution is combined with a solution containing a source of phosphorus, a metal phosphate containing aluminum may be formed during a heat-set treatment. Thus, for a surface of an iron-containing metal composition pits or valleys of a heterogeneous surface (such as a brushed stainless steel) may be filled with material including an iron phosphate after heat setting.

An aspect of this invention is forming a metal surface, such a surface of brushed stainless steel, that resists fingerprint or other stain retention as well as resisting tarnishing at elevated temperatures such as up to 400 or 450° C.

A further aspect of the invention is to maintain the appearance of an uncoated surface such as the surface of a brushed stainless steel. Another aspect of this invention is that, typically, a heat-set layer applied to a metal surface is transparent to visible light such that a coated or treated surface retains the appearance of an uncoated surface. Another aspect of this invention is to provide a surface treatment or coating that maintains protective properties after bending, machining, or forming an article from a coated metal sheet.

Another aspect of this invention is that a metal surface suitable for treatment according to this invention may be as received from a mill or may have been in use and had acquired surface contaminants.

In another aspect of this invention, a surface treatment of this invention may be applied to a metal surface that has been subjected to conditions in which solid residue has accumulated on the surface from service. Such surfaces, which may be interiors of pipes that have been used in natural gas or petroleum refining environments, may be treated in accordance with this invention to provide stable, heat-set reaction product between the metal surface containing the residue and a phosphate precursor material and is better capable of withstanding harsh service environments.

A further aspect of this invention is forming one or more secondary phospho alumina coating layers on top of a metal substrate surface coated with a primary layer.

Another aspect of this invention is a brushed or heterogeneous metal surface that has been coated or treated with a primary layer of this invention such that valleys contained in such surface are filled with an inorganic metal phosphate material including a metal phosphate material containing aluminum. Typically, the extent of filling of such material should be sufficient to form a surface in the valleys that will demonstrate the performance desired such as to resist stain or fingerprint retention or is sufficient to form a surface that provides a primary surface onto which one or more secondary phospho alumina surfaces may be applied. Depending on the level of performance desired, the valleys may be filled at levels at least 25%, 30%, 40%, 50%, 75% or 85% by volume.

Another aspect of this invention is control over the extent of reaction between metal and phosphoric acid, phosphate ester, or other phosphating source by carefully controlling the quantity of reactive species available for reaction. By control of concentration of reacting species in solution and the wet film thickness on a metal surface (including microstructure valleys within the surface) during deposition, a predetermined quantity of reactive material is deposited on the metal. The reaction may be driven to completion through the heat-setting process. Typical phosphating processes do not yield thermally stable phosphate materials. The present invention is ideal for use in conventional coil coating facilities with only minor modifications of equipment or conditions.

Other aspects of this invention include articles made from metal such as a brushed metal, preferably brushed stainless steel.

A further aspect of this invention is using a lower grade stainless steel that has been treated according to this invention in environments for which higher grade stainless steels are useful.

Another aspect of this invention treating a metal surface with a heat-set phosphoric acid-containing solution or a phosphate ester-containing solution, optionally followed by forming secondary layers of phospho aluminas aluminas, silicas, or silica aluminas. A further aspect of this invention is a controlled formation of metal phosphate reaction product on a metal surface (such as in valleys on the surface) at elevated temperature (e.g., >200 or >250° C.).

Further aspects of the invention include producing a coated article using the method described in this invention and producing a coated or treated article by coil coating in a continuous process. A thin (typically <2 micrometers, average), flexible inorganic layer applied in accordance with this invention is suitable for coil coating.

Also, thermal curing of coated articles made according to this invention may yield a hydrophobic or hydrophilic surface that can be exploited suitably for a non-stick or stain resistance properties.

NMR analysis indicates that phosphorus in a solution formed by adding phosphoric acid to ethanol exists primarily as phosphoric acid with a minor component of phosphate ester, although such a mixed solution may form a phosphate ester during heat processing as described below. However, a solution formed from a phospho-alumina precursor (as described below) in which phosphoric acid is added shows primarily phosphate esters with little free phosphoric acid. Solutions containing primarily phosphate esters may be preferable to avoid use of highly acidic formulations.

Phosphoric acid useful in this invention typically is 85 wt. % phosphoric acid with minimum water content, although other suitable grades may be used.

A useful phosphorus-containing solution should provide sufficient amounts of phosphorus species to react with exposed metal on the metal surface. A typical initial solution contains, or is derived from, 0.5 to 50 vol. %, typically 1 to 20 vol. % or 2 to 10 vol. %, phosphoric acid (as 85 wt. % acid) or derivatives in the solvent.

A preferable suitable solvent is a substantially (>50 vol. %, preferably >75 vol. %) non-aqueous liquid that is compatible with a phosphate ester-containing material is capable of wetting a metal surface to form a uniform layer of solution onto the metal surface. A aqueous-based solution with suitable surfactants may be useful. An example of a suitable solvent contains a C₁-C₄ alcohol, such as methanol, ethanol, n-propanol, isopropanol, butanol, or mixtures thereof. A preferable alcohol solvent is ethanol.

A phosphate ester-containing solution also may contain a precursor material to phosphorus-enriched aluminas or phospho-aluminas. As used in this invention, these phospho aluminas may be made typically by forming a precursor solution containing a complex mixture of aluminum salt and phosphate esters, which is cured by heating to a temperature sufficient to form a phospho alumina. These sol-gel precursor materials typically are formed in a non-aqueous liquid such as an alcohol (typically a C₁-C₈ alcohol or mixtures thereof and preferably ethanol).

In a typical procedure, ethanol solutions of an aluminum salt (such as aluminum nitrate) and a phosphorus oxide such as phosphorus pentoxide (P₂O₅) or phosphorus oxide ester are combined in aluminum to phosphorus atomic ratios suitable for mixing with a phosphate ester-containing solution. In a typical resulting solution, less than 5% or even 1% of the phosphorous is retained as phosphoric acid as indicated by NMR. Much of the remaining phosphorous is present in the form of phosphate esters or complex phosphate species or in phospho alumina species. Such a resulting solution mixture may be more practical or suitable for use as a solution to produce a first or primary layer. Even in this case, the total phosphorous content in the dried first layer is sufficient to form a metal phosphate layer by reacting with the substrate during heat setting of the first layer. Typical aluminum to phosphorus ratios in a phospho alumina precursor useful in forming the primary layer are at least 0.5 or above and typically are up to 2 or above. Typically, concentration of the phospho-alumina material in the liquid is about 0.01 to about 1.5 molar (M) (based on aluminum) and typically about 0.05 to 0.5 M. Preferably, the precursor solution and the resulting coating materials are halide free. Examples of phospho-alumina coating systems are described in U.S. Pat. Nos. 6,036,762, 6,461,415, 7,311,944, 7,678,465, 7,682,700, and 8,021,758, all incorporated by reference herein. In this invention, phosphoric acid or phosphate ester may be added to such a phospho alumina precursor system to form a liquid precursor that is first applied to a metal surface as a primary layer.

In an aspect of this invention, addition of phosphoric acid or phosphate ester solution to the phospho alumina precursor solution may produce a mixture with an aluminum to phosphorus ratio more than 0, at least 0.05, at least 0.1, at least 0.15, at least 0.2, at least 0.3 or at least 0.4. Such solution may have typical aluminum to phosphorus ratios up to 0.7, up to 0.5 or up to 0.4. Such a mixed solution forms a stable material with a low aluminum to phosphorus atomic ratio that may be used to form a primary layer on a metal surface on which secondary layers of phospho alumina with higher aluminum to phosphorus atomic ratios may be placed. Adding phosphoric acid to a phospho alumina precursor solution to reach a particular low aluminum to phosphorus ratio produces a chemical species that is distinct from making a phospho alumina precursor with such low aluminum to phosphorus ratio.

Preferably to form a coated substrate of this invention, a liquid layer of a phosphate precursor solution such as a phosphate ester-containing solution is applied to a metal surface, and that liquid layer is dried and heat set onto the surface to form a primary layer. Thus, the liquid phosphate ester-containing material (which itself may contain precursors to phosphate esters) is applied to a metal surface. This solution may be applied to a metal surface by any suitable means such as dip, roll, spray, flow or spin processes either in batch or continuous coil coating processes. In accordance with this invention, the liquid phosphate ester precursor solution layer is dried (typically below 150° C.) and heat set at a time and temperature sufficient to provide an initial, primary protective layer onto the surface, which, in the case of a brushed surface, may be preferentially formed within the valleys in the brushed surface. Typically, the heat set temperature is at least 200° C., preferably at least 250° C., may range up to 450 or 500° C. or more. A suitable time may be 1 minute or less, at least 2 minutes, at least 5 minutes, at least 10 minutes, at least 15 minutes, at least 20 minutes, at least 30 minutes, or more. Typical heat set conditions include exposing a metal surface coated with a liquid phosphate ester-containing material for 2 to 90 minutes at 200 to 400° C. or more. Typically, the higher the heat set temperature, the less time is required. Thus, heat setting at a temperature of 400° C. may require only a 2-minute exposure or a heat set temperature of 900° C. may require only 10-15 seconds to form a metal phosphate reaction product. Other typical conditions include 20 to 60 minutes at 225 to 275° C.; 15 to 45 minutes at 225 to 300° C.; 10 to 60 minutes at 250 to 300° C.; and 30 to 60 minutes at 200 to 250° C. Heat setting may occur under normal atmospheric conditions or may occur under pressure or vacuum under dry or moist conditions. Heating times and temperatures may be adjusted according to the thermal mass of an object with a metal surface and heat exchange rates in a heating device. It is believed that heat setting transforms a phosphate precursor such as a phosphate ester-containing solution to a solid, inorganic metal phosphate reaction product onto an iron-containing metal surface. Such reaction product may partially (such as at least 25%, 30%, 40%, 50%, 75% or 85% by volume) or completely fill valleys of a heterogeneous (such as a brushed) metal surface. A typical brushed stainless steel surface of this invention has at least 50 vol. % of valleys filled with metal phosphates.

After drying and heat setting, the surface coated with the phosphate ester-containing solution does provide a level of protection against stain and fingerprint retention and tarnish creation at moderate temperatures to some metal surfaces. However, preferably for tarnish protection at higher temperatures such as above 250° C., at least one additional layer of a phospho-alumina is coated onto the surface with a primary layer. Also, layers of other materials may be applied to a surface coated with such heat-set phosphate ester-containing solution, such as alumina, silica, and silica alumina.

Treatment or “phosphating” of a metal surface with phosphoric acid or solutions or slurries of metal (e.g., iron, zinc, or manganese) salts in phosphoric acid before further use is well known. Conventional phosphating is conducted at near ambient temperatures under acidic conditions. However, phosphating a metal surface with a conventional phosphating agent such as phosphoric acid prior to coating with a phosphorus-enriched alumina in a manner described in this invention does not provide a stable, long lasting surface as provided using the treatment of this invention. Although coatings of this invention are believed to contain chemically-bonded metal phosphate linkages at the substrate surface, a phosphating treatment with phosphoric acid under conventional conditions that do not include a heat set of a phosphoric acid containing solution onto a metal surface does not provide the type of surface bonding as created using heat-set application of a phosphate ester or to a further phospho-alumina layer according to aspects of this invention. In the present invention, a stable, typically dry, phosphorus-rich material is formed, and a phosphating reaction occurs upon further thermal curing to form a metal phosphate filler in microstructure valleys together with a phosphate reaction product on the plateaus of a metal surface. An advantage of this invention is an ability to form thin, smooth uniform layers of phosphate material in a controlled manner such that the original aesthetic properties are maintained while providing environmental protection. Typically, a suitable phosphate precursor solution does not contain metal ions, such as iron, zinc, or manganese, and metal phosphate reaction product of this invention is formed from a reaction between metal atoms in the metal surface and a reactive species (such as phosphoric acid or a phosphate ester) in the phosphate precursor solution or the dried residue thereof.

A secondary phospho-alumina layer may be applied to the primary layer to form a coating system of this invention. A phospho-alumina precursor material component useful in this invention may be formed over a wide range of aluminum to phosphorous ratios. Such secondary phospho-alumina layer typically has a substantially higher Al/P atomic ratio compared to the primary layer and typically has an Al/P atomic ratio of at least about 1 and more typically greater than or equal to about 2. The Al/P atomic ratio may range up to 10 or more and typically is up to about 4. A typical secondary phospho-alumina layer has an Al/P atomic ratio of about 1 to 10 and may be about 2 to about 8. Additional secondary layers may be applied, each typically with higher Al/P ratios than may exist in the primary layer.

Individual coating layers usually may be about 0.05 to 0.5 micrometer and typically about 0.1 to 0.3 micrometer. When applied to a brushed surface having abrasions, the primary coating becomes a filler of the valleys of a heterogeneous metal surface and the secondary phospho alumina layer provides a stable, chemically uniform, and substantially smooth surface. Coating thickness should be tailored such that the coating provides the desired surface properties while not too thick to modify the visual appearance of the surface or to limit flexibility of the substrate such as in coil coating production or formation of articles. Typically, the coating also is transparent in the visible spectrum. The phosphated valleys contain mostly metal phosphate (typically amorphous), which is stable to higher temperatures. The thermal stability and chemical inertness of the metal phosphate may be enhanced by addition of aluminum or other metal constituents in the first phosphate ester mixture solution. Thus the first inorganic filler layer will be transparent and dense. For the primary layer containing a phospho alumina component, the source of phosphorus can be any phosphorus-containing species that is soluble in a solvent media with concentration high enough such that the resulting metal phosphate formed after curing in the valleys is sufficient to fill them. Sources may include solutions and mixtures of phosphoric acid, phosphate esters, organic phosphates, metal phosphate salts, and similar groups. Addition of metal salts to alcoholic solutions of phosphoric acid creates an ester mixture solution with very little or no free strong acid and which is stable and more environmentally-friendly for processing. Such metal salts can include metal from Groups 3-13. A most preferred metal salt is aluminum nitrate.

Typically, a secondary phospho-alumina layer on the primary coated metal surface is formed by applying a precursor solution containing aluminum salt and a phosphorus oxide or ester followed by drying and curing to a suitable temperature. Each secondary layer of phospho-alumina material may be applied to a substrate through a precursor material with a similar Al/P atomic ratio by techniques such as dip-coating, spray coating, roll coating, or spin coating, either in batch or continuous coil coating processes. Additional secondary coatings may have different Al/P atomic ratios. Typically, the secondary coating layers may be deposited as a dense, pinhole-free thin coating on substrates at relatively low temperatures such as room temperature or slightly above. Preferably, a uniform layer of precursor is applied followed by curing at an elevated temperature, which typically is above about 400° C., but may range above 250 to 350° C. Typically, curing may be accomplished at above 500° C. and may range up to 1000° C. or more. Curing time may range from a few seconds to several hours. Typically, a precursor layer is cured at 400 to 900° C. for 10 seconds to 15 minutes to form the coating material used in this invention. A precursor layer may be dried initially at 100 to 150° C. to remove volatile components and yield a solid. Some loss of phosphorous from the initial higher phosphorus-containing layer is expected upon exposure to higher temperatures. One or more secondary layers may be applied in sequential steps. A formed heat-set layer may be rinsed with a suitable solvent such as water or alcohol between such steps.

Secondary phospho-alumina coatings used in this invention typically are highly inert to chemical attack, and typically are thermally stable at high temperatures. High temperature oxidation tests have shown that these coatings also are highly impervious to gas (e.g. oxygen) transmission. As a highly covalent inorganic oxide, the coatings are chemically inert (like alumina) and thermally stable materials. Testing of the coating materials has demonstrated electrical insulating properties and continuity, hermeticity and, protective nature of the coating. These phospho-alumina terminated surfaces typically will have improved surface properties to enable ease of cleaning in routine use.

Layers of phosphorus-enriched aluminas or phospho-aluminas used in this invention may be made typically by forming a sol-gel containing oxides of aluminum to which oxides of phosphorus are incorporated, which is cured by heating to a temperature sufficient to form a phosphate of one of the metal oxide constituents on the surface. These sol-gel precursor materials typically are formed in a non-aqueous liquid such as an alcohol (typically a C₁-C₈ alcohol or mixtures thereof and preferably ethanol) in not strong acid conditions (pH >2). In a typical procedure, solutions of an aluminum salt (such as aluminum nitrate) and a phosphorus oxide such as phosphorus pentoxide (P₂O₅) or phosphorus oxide ester are combined in aluminum to phosphorus atomic ratios suitable for creating a desired layer of material. Typically, concentration of the phospho-alumina material in the liquid is about 0.1 to about 1 molar (M) and typically about 0.2 to 0.6 M. Preferably, the coating materials are halide free. Examples of phosphorus-alumina coating systems are described in U.S. Pat. Nos. 6,036,762, 6,461,415, 7,311,944, 7,678,465, 7,682,700, and 8,021,758, all incorporated by reference herein.

The principal structural components of the phospho-alumina precursor solutions useful for secondary layers used in this invention appear to be complexes that contain Al—O—Al linkages.

Metal surfaces treated in accordance with this invention exhibit fingerprint and stain resistance as well as resistance to corrosion such as by oxidation in environments that include harsh environments of temperature and contact with corrosive materials. Also, metal surfaces, especially brushed metal surfaces, show resistance to tarnish at elevated temperatures such as up to 500° C. or above as well as resistance to retention of stains or marks applied to such surfaces. Typically, many stains or marks may be removed by wiping with tissue or cloth, which may be dry or wet, cool or heated, or using a soap or detergent. If required, organic solvents, such as ketones (e.g., methyl ethyl ketone or acetone) or hydrocarbons such as paint thinner may be used to clean a surface of this invention without substantial loss of surface appearance. A metal surface, especially a brushed metal surface, will retain an original luster appearance and magnetic properties after treating or coating in accordance with this invention. Other advantages of this invention include providing a galvanic corrosion resistance caused by contact of different metal materials. Also, a mirror finish may be applied to a brushed metal coated in accordance with this invention by applying a reflective layer such as aluminum applied by thermal evaporation to all or part of a coated brushed metal surface formed in accordance with this invention.

Typical treated substrates of this invention do not discolor upon uv exposure or application of organic solvents such as used in typical cleaning operations. Also, typical surfaces of this invention may be subject to some abrasion without loss of protective or appearance properties.

In a further aspect of this invention, a metal such as a mild steel may be first coated with a heat-set phosphate ester-containing solution or phosphoric acid-containing material followed by one or more coats of a phospho alumina such as formed from heat curing a sol-gel precursor solution as described. A mild steel coated with primary and secondary layers according to this invention exhibits better oxidation resistance under adverse conditions in contrast to a mild steel coated only with a phospho-alumina layer.

Examples of articles coated in accordance with this invention include stainless steel products, such as cooktops, ovens, cooking surfaces, refrigerator panels, sinks, grills, countertops, facades, walls, doors, structural and architectural panels, garbage cans, automotive, aircraft, bus, and train metal panels, fireplace fronts and tools, signs, displays, writing boards, sculptures, and the like. Other uses are for metal ball bearings and glides. Typically, objects with metal surfaces that may be subject to staining from external sources such as markers or graffiti may use the protective coating system of this invention. Such marks or stains typically may be removed easily with mild wiping or cleaning from such surfaces.

Aspects of the invention are illustrated but not limited by the following examples.

Example 1

A coating precursor solution was prepared by adding 264 grams of aluminum nitrate nonahydrate (GFS Chemicals, Powell, Ohio) to 300 milliliters of anhydrous ethanol. In a separate container, 25 grams of phosphorus pentoxide (Sigma Aldrich, St. Louis, Mo.) were dissolved in 100 milliliters of anhydrous ethanol and then the two solutions were combined to yield a solution with a 2 to 1 atomic ratio of aluminum to phosphorus, and the resultant solution was diluted with ethanol to yield a solution with an aluminum concentration of 0.89 Molar. The resulting solution was placed in a round-bottom flask fitted with a condenser and refluxed for 16 hours. The refluxed solution was concentrated in a rotary evaporator with bath temperature of 70° C. and reduced in volume by one half. Ethanol was then added to the resulting solution to yield a solution with an aluminum concentration of 1.0 Molar and an aluminum to phosphorus atomic ratio of 2 to 1.

A 150-milliliter portion of the resulting solution was mixed with 812.5 milliliter of ethanol and then stirred continuously while 37.5 milliliter of 85% phosphoric acid was added slowly to form a clear and stable solution having a calculated Al/P atomic ratio of 0.24. A 400 MHz ³¹P NMR spectrum of the solution showed broad peaks at 0.396 ppm and −12.058 ppm.

Example 2

A 3-inch×4-inch (7.6 cm×10.2 cm) coupon of 304 stainless steel sheet 0.048 inch (1.2 mm) thick with a brushed #4 satin finish was immersed in the solution of Example 1 for two minutes and slowly withdrawn from the solution at a rate of 6 inches (15 cm) per minute. The sample was dried under warm air and then placed in a furnace at 400° C. for 2 minutes. The resulting coated sample had a similar metallic luster as the uncoated sample and was free from iridescence or tarnish colors.

Example 3

A second coating precursor solution was prepared by adding 264 grams of aluminum nitrate nonahydrate (GFS Chemicals, Powell, Ohio) to 300 milliliters of anhydrous ethanol. In a separate container, 25 grams of phosphorus pentoxide (Sigma Aldrich, St. Louis, Mo.) were dissolved in 100 milliliters of anhydrous ethanol and then the two solutions were combined to yield a solution with a 2 to 1 atomic ratio of aluminum to phosphorus, and the resultant solution was diluted with ethanol to yield a solution with an aluminum concentration of 0.89 Molar. The resulting solution was placed in a round-bottom flask fitted with a condenser and refluxed for 16 hours. The refluxed solution was concentrated in a rotary evaporator with bath temperature of 70° C. and reduced in volume by one half. Ethanol was then added to the resulting solution to yield a solution with an aluminum concentration of 0.2 Molar and an aluminum to phosphorus atomic ratio of 2 to 1.

Example 4

A coupon of 304 stainless steel sheet with a brushed #4 satin finish was prepared according to Example 2. The coated coupon then was immersed in the solution of Example 3 for two minutes and slowly withdrawn from the solution at a rate of 6 inches per minute. The sample was dried under warm air and then placed in a furnace at 400° C. for 2 minutes. The prepared sample had a similar metallic luster as the uncoated sample and was free from iridescence or tarnish coloration.

Comparative Run A

A 3 inch×4 inch (7.6 cm×10.2 cm) coupon of 304 stainless steel sheet 0.048 inch (1.2 mm) thick with a brushed #4 satin finish was coated only with the solution of Example 3. The resulting coated sample showed distinct coloration compared to the uncoated sample

Example 5

A coated coupon prepared according to Example 2 and an uncoated coupon of the same brushed #4 satin finish 304 stainless steel were placed in an oven at 450° C. for one hour in air and then removed directly to room temperature. The uncoated coupon subjected to the heat treatment had a dark yellow coloration. The sample prepared according to Example 2 and subjected to the heat treatment retained the original silver color and metallic luster of the uncoated stainless steel.

Example 6

A brushed 304 stainless steel coupon prepared according to Example 2 was folded in a vice to form a bend diameter of approximately 1 cm. The folded coupon was then placed in an oven at 450° C. for one hour in air and then removed directly to room temperature. The sample retained the original silver color and metallic luster of the uncoated stainless steel in all regions including regions with minimum bend radius and within impressions made in the steel by the vice jaws.

Example 7

A brushed 304 stainless steel coupon was prepared according to Example 4. The coated coupon again was immersed in the solution of Example 3 for two minutes and slowly withdrawn from the solution at a rate of 6 inches (15 cm) per minute. The coupon was dried under warm air and then placed in a furnace at 400° C. for 2 minutes. The prepared coupon sample had the same metallic luster as the uncoated sample and was free from iridescence coloration. The sample was also smoother to the touch and showed a 25% reduction in surface roughness measured by atomic force microscopy (AFM).

Fingerprints were placed on the coated sample and an uncoated stainless steel coupon. The fingerprints were highly visible on the uncoated sample; whereas, the fingerprints on the coated sample were less visible. The coupons were then wiped with dry tissue (Kimwipe®) five times. The fingerprints on the coated sample were no longer visible; however, the fingerprints on the uncoated sample were still highly visible.

Example 8 and Comparative Run B

A coated brushed #4 satin finish 304 stainless steel coupon (Example 8) was prepared according to Example 2 except that the coating precursor solution was a 5 vol. % solution phosphoric acid in ethanol. This coated coupon was dried in warm air and placed in an oven at 450° C. for one hour in air and then removed directly to room temperature. The coated coupon sample prepared using a phosphoric acid in ethanol precursor solution and subjected to drying and heat treatment retained the original silver color and metallic luster of the uncoated stainless steel. The sample area that was in contact with the solution retained the silver color and metallic luster of the as-received brushed stainless steel; whereas, the sample area that was not in contact with the solution was distinctly yellow/wheat colored. A comparative coupon (Run B) was prepared as described above, but before drying and heat treating, the coupon was rinsed with Millipore-filtered water. After heat treatment, the entire coupon showed distinct yellow/wheat coloration. The sample area that was in contact with the solution had a slightly lighter coloration than the area that had not come in contact with the solution.

Example 9

A series of experiments were performed in which a brushed stainless steel coupon was coated using procedures according to this invention but under differing time and temperature conditions. In these experiments a coated brushed #4 satin finish 304 stainless steel coupon was prepared according to Example 2 except that the coating precursor solution was a 5 vol. % solution phosphoric acid in ethanol. This coated coupon was dried in warm air and placed in an oven in air for a determined temperature and time and then rinsed with Millipore-filtered water. The coupon then was dried and heated to 450° C. for one hour in air.

When the precursor coating was heated at 200° C. for 10 minutes, the area in contact with the solution showed variation in coloration from a majority of distinct yellow/wheat to small regions of blue/metallic. Tarnish was very non-uniform spatially. The least tarnished areas displayed blue/silver metallic coloration compared with untarnished 304 stainless steel.

When the precursor coating was heated at 250° C. for 10 minutes, the sample area that was in contact with the solution predominantly retained the silver color and metallic luster of the as-received brushed stainless steel; whereas, the sample area that was not in contact with the solution was distinctly yellow/wheat colored.

When the precursor coating was heated at 200° C. for 30 minutes, a substantial portion of the region that was in contact with the solution retained the silver/metallic coloration of uncoated stainless steel with isolated blue regions; upper and lower edges of the region in contact with the solution exhibited distinct yellow/wheat coloration.

When the precursor coating was heated at 200° C. for 60 minutes, the majority of the region that was in contact with the solution showed silver/metallic coloration with a brown shift compared to uncoated stainless steel. Isolated blue regions occurred around the edges.

Example 10

A 3-inch×4-inch (7.6 cm×10.2 cm) coupon of 304 stainless steel sheet 0.048 inch (1.2 mm) thick was prepared according to Example 2 except that the stainless steel sheet was polished. After the sample coupon was withdrawn from the precuror solution, the solution film on the surface pooled (de-wet) forming a non-uniform film. Following heat treatment, the sample area that had been in contact with the solution exhibited blue spots approximately ⅛ inch (3 mm) in diameter distributed across the surface. The sample area that had not been in contact with the solution was a deep yellow/wheat color.

Example 11

A coated coupon was prepared according to Example 8, except that the coupon was #4 satin finish 430 stainless steel and the coated coupon was heated at 450° C. for 30 minutes. The majority of the sample area that was in contact with the solution retained the silver color and metallic luster of the as-received brushed stainless steel; whereas, the sample area that was not in contact with the solution was distinctly yellow/wheat colored.

Example 12

A 2-inch×3-inch (5.1 cm×7.6 cm) coupon of mild steel sheet 0.030 inch (0.77 mm) thick with a mill finish was immersed in a 7.5 vol. % solution phosphoric acid in ethanol for two minutes and slowly withdrawn from the solution at a rate of 6 inches (15 cm) per minute. The sample was dried under warm air and then placed in a furnace at 500° C. for 1 minute. The resulting coated sample had a similar metallic luster and appearance as the uncoated sample.

Example 13

A third coating precursor solution was prepared by adding 264 grams of aluminum nitrate nonahydrate (GFS Chemicals, Powell, Ohio) to 300 milliliters of anhydrous ethanol. In a separate container, 49.7 grams of phosphorus pentoxide (Sigma Aldrich, St. Louis, Mo.) were dissolved in 100 milliliters of anhydrous ethanol and then the two solutions were combined to yield a solution with a 1 to 1 atomic ratio of aluminum to phosphorus, and the resultant solution was diluted with ethanol to yield a solution with an aluminum concentration of 0.89 moles per liter. The resulting solution was placed in a round-bottom flask fitted with a condenser and refluxed for 16 hours. The refluxed solution was concentrated in a rotary evaporator with bath temperature of 70° C. and reduced in volume by one half. Ethanol was then added to the resulting solution to yield a solution with an aluminum concentration of 0.3 moles per liter and an aluminum to phosphorus atomic ratio of 1 to 1.

A coupon of mild steel sheet with a mill finish was prepared according to Example 12. The coated coupon then was immersed in the third coating precursor solution for two minutes and slowly withdrawn from the solution at a rate of 12 inches (30 cm) per minute. The sample was dried under warm air and then placed in a furnace at 500° C. for 1 minute. The resulting coated sample had a similar metallic luster and appearance as the uncoated sample.

Example 14

A coupon of mild steel sheet with a mill finish was prepared according to Example 12. The coated coupon then was immersed for two minutes in a coating precursor solution prepared according to Example 3 except that the coating precursor solution had an aluminum concentration of 0.5 moles per liter. The sample was slowly withdrawn from the solution at a rate of 12 inches (30 cm) per minute. The sample was dried under warm air and then placed in a furnace at 500° C. for 1 minute. The resulting coated sample had a similar metallic luster and appearance as the uncoated sample.

Comparative Run C

A 2-inch×3-inch (5.1 cm×7.6 cm) coupon of mild steel sheet 0.030 inch (0.77 mm) thick with a mill finish was coated only with the coating precursor solution of Example 13. The resulting coated sample showed distinct coloration compared to the uncoated sample.

Example 15

Two coated mild carbon steel coupons were prepared according to Example 13 and Example 14. The coated mild steel coupons were placed in a salt fog chamber according to ASTM B117 along with an uncoated piece of mild steel. After eight hours of exposure, the two coated samples showed significantly less corrosion than the uncoated sample.

Example 16

A coupon of 304 stainless steel sheet with a brushed #4 satin finish was prepared according to Example 4. The coated piece and an uncoated piece of brushed #4 satin finish were marked with an Expo® dry erase maker containing methyl isobutyl ketone. The ink was allowed to dry for a few seconds and was wiped once with a dry Kimwipe®. The ink on the coated material was completely removed while the uncoated material showed highly visible streak marks.

In a similar fashion, the same piece of coated material was marked with an Expo® dry erase maker and allowed to dry for 24 hours. The ink was then wiped once with a dry Kimwipe®. The ink was completely removed with no signs of staining or residue. 

What is claimed is:
 1. A method of coating a surface of an iron-containing material, the process comprising: applying a first layer that comprises a phospho alumina precursor solution that has an aluminum to phosphorous atomic ratio of 0.1 to 0.7 to the surface; forming an iron-phosphate precursor on the surface; drying the first layer; and then heat setting the first layer by heating to a temperature above 200° C.
 2. The method of claim 1, wherein the first layer comprises a phospho alumina precursor solution that has an aluminum to phosphorous atomic ratio of 0.1 to 0.5.
 3. The method of claim 1, wherein the surface has a heterogeneous surface microstructure that includes valleys and peaks; wherein the first layer fills valleys in the heterogeneous surface microstructure.
 4. The method of claim 3, wherein the surface has an average roughness (Ra) of 0.2 to 1.5 microns.
 5. The method of claim 4, wherein the heat set first layer has a thickness of about 0.05 to 2 microns.
 6. The method of claim 5, wherein the surface has an Ra of 0.4 to 0.6; and the heat set first layer has a thickness of about 0.1 to 0.3 microns.
 7. The method of claim 5, wherein, after heat setting, the valleys are filled with an iron phosphate.
 8. The method of claim 3, wherein the surface is a brushed surface.
 9. The method of claim 1 further comprising applying a second layer that comprises a phosphor-alumina precursor solution which has an aluminum to phosphorous ratio of about 1 to 10 to the heat set first layer; drying the second layer; and then heat setting the second layer at a temperature above 250° C. to 1000° C.
 10. The method of claim 9, wherein the second layer comprises a phosphor-alumina precursor solution that has an aluminum to phosphorous ratio of about 2 to
 8. 11. The method of claim 9, wherein the surface is a brushed surface; wherein the brushed surface has an average roughness (Ra) of 0.2 to 1.5 microns; and wherein the second layer provides a chemically uniform and substantially smooth surface.
 12. The method of claim 11, wherein the heat set first layer has a thickness of about 0.05 to 0.5 microns; wherein the heat set first layer fills valleys in the brushed surface; and wherein the heat set second layer has a thickness of about 0.05 to 0.5 microns.
 13. The method of claim 12, wherein the brushed surface has an Ra of 0.4 to 0.6; and the heat set first layer has a thickness of about 0.1 to 0.3 microns.
 14. The method of claim 1, wherein the iron-containing material is stainless steel. 